Resetting a theme– how to do it?

I’ve installed several themes and tried them out with my existing blog. After customizing a few I wanted to start over on a couple and am not able to do so, even after deleting the entire theme’s folder in my wp-content folder on my server. It’s like the css style sheet remains the one I customized, even when I install a fresh clean version of the theme. What the?? Is there another style sheet somewhere in another folder I don’t know about? Outside of de-installing the entire wordpress app from my server and re-installing it clean, what can I do?

Thanks anyone. Appreciated.